This invention relates to a membrane separation device. In particular, this invention relates to a hollow fiber membrane device for liquid separations designed to accommodate significant changes in fiber dimensions due to fiber swelling or contraction during operation.
Membrane devices are used in a wide variety of separation applications such as reverse osmosis, ultrafiltration, microfiltration, dialysis, and pervaporation. Different membrane device configurations are described in the art, including plate and frame, tubular, spiral wound, and hollow fiber configurations. The hollow fiber configuration is generally preferred because a higher surface area per unit volume of device can be obtained, resulting in increased device productivity over other configurations. Furthermore, hollow fibers are generally self-supporting so the hollow fiber configuration does not require the extensive membrane supporting means and flow channel spacers used in plate and frame or spiral configurations.
Hollow fiber membrane devices are typically fabricated according to one of two basic designs, parallel wrapped or bias wrapped. The typical parallel wrapped hollow fiber device consists of a plurality of hollow fibers arranged in parallel in an axial direction, optionally about a core, with the ends of the hollow fibers embedded in at least one tubesheet to form a bundle. The bundle is inserted into a case against which the tubesheet(s) seals so as to define two fluid regions, one for the permeate and one for the non-permeate. Fluid flow between the two regions is accomplished by fluid selectively permeating through the membranes. The case has means for introducing feed to the membranes and means for removing permeate and non-permeate from the membranes. Examples of such parallel wrapped devices are described in U.S. Pat. Nos. 3,690,465; 4,220,535; 4,265,763; 4,271,900; 4,315,819; 4,367,139. The typical bias wrapped device consists of a plurality of hollow fibers wrapped in a helical or spiral fashion about a core to form multilayers of criss-crossed fibers. A bundle is thus formed with at least one end of the bundle embedded in a tubesheet. The bundle is fitted into a case against which the tubesheet(s) seals, forming two separate fluid regions. The case contains menas of introducing the feed and withdrawing the permeate and non-permeate. Examples of such bias wrapped devices are described in U.S. Pat. Nos. 3,422,008 and 4,430,219.
In the conventional hollow fiber devices hereinbefore described, the fibers are wrapped under sufficient tension to hold the fibers relatively fixed in place during operation. In applications in which the hollow fibers are contacted with a liquid which causes significant dimensional changes in the fibers, the hollow fibers may crimp and/or break due to dimensional changes caused by swelling or contraction. Changes in fiber diameter of up to about 100 percent and changes in fiber length of up to about 100 percent are not uncommon when the fibers are contacted with a fluid which swells or contracts the fibers. Furthermore, reduced flow rates within the follow fibers and high pressure drops down the hollow fibers may result from the swelling shut of the fibers, since both the inside and outside wall diameters may change in dimension.
A device design which will accommodate large changes in fiber diameter and length without crimping or breaking of fibers is needed.